1. Field of the Invention
This invention relates generally to a method for producing diamond by chemical vapor deposition.
2. Description of the Prior Art
Diamond has long been sought after not only for its intrinsic beauty and value as a gemstone but for its many unique and valuable mechanical, electrical, optical, and thermal properties. Diamond is the hardest material occurring in nature, has a low coefficient of friction, and is extremely resistant to chemical attack. It also is classified as a high bandgap semiconductor, is optically transparent to much of the electromagnetic spectrum, and has the highest heat conductivity of any material at room temperature. While, in fact, naturally occurring diamonds are far from scarce, humankind has long sought to produce these crystals synthetically.
The first such syntheses involved the application of high pressures and temperatures to bring about the allotropic transformation of graphite to diamond (for example, see U.S. Pat. No. 2,947,608). More recently, numerous studies have shown that diamond can be produced synthetically at low pressures using various forms of chemical vapor deposition (CVD) processes employing a gaseous carbon compound (for example, see U.S. Pat. Nos. 3,030,187, 4,767,608, and 4,873,115).
In all of the CVD processes to date, with the exception of CVD growth on diamond single crystals where the growth is epitaxial for thin layers, the diamond produced takes the form of a thin polycrystalline film of extremely small diamond particles (typically less than 100 .mu.m in diameter). In many of the processes, diamond is not formed by itself, but rather in combination with graphite and diamond-like carbon (the latter species being a carbon allotrope with properties between those of aphite and diamond). The prior art processes for producing diamond all employ some high energy method of pretreating or activating one or more of the reactant species such as microwave or rf-generated plasmas (plasmas being the mixture of electrons and gaseous ions formed when the gases are heated to the range of 5,000.degree. C. to 30,000.degree. C.), or hot filaments, high temperature flames, arc discharges, electron beams, lasers, etc. (which heat the gases to a temperature of 2,000.degree. C. to 3,000.degree. C. or higher). All such high-temperature, high-energy steps comprise methods of pretreatment of said gases to activate them to a high energy level. The activated gases then are impinged upon a substrate, with diamond growth occurring principally on the substrate surfaces directly in the path of the activated gases or plasma. Such processes are expensive because of the energy costs of activating the reactant gases and are relatively low-volume because of the difficulties of activating large volumes of gases. Also, they make it difficult to coat three dimensional or irregular objects with diamond film, because the objects must be turned to expose successive sides or areas to the flow from the activated gases.
Most CVD processes also occur at pressures less than 100 Torr; those which do not, typically produce very impure diamond/graphite mixtures.
Many potential markets exist for diamond films and may involve the use of diamond coatings for extreme hardness, inertness to chemical attack, heat conductance, and other desirable properties. Some applications may further use doped diamond for its unique electrical properties. A major drawback to presently existing CVD diamond coating technologies is the difficulty of placing a diamond film uniformly on objects with complex shapes. Another major problem exists with the high temperatures (typically &gt;700.degree. C.) usually required for diamond formation in existing CVD processes. Our invention eliminates many of these problems.
In application Ser. No. 528,804 there is described a method for depositing CVD diamond films and particles employing a halogen-assisted technique. While this technique is unique and solves many of the problems associated with CVD diamond deposition, it has been found that diamond deposition may be enhanced further by the addition of a chalcogen to the process of application No. 528,804.
Chalcogens comprise the elements oxygen, sulfur, selenium, tellurium and polonium from group VI of the periodic table. As will be appreciated by those skilled in the art, a chalcogen (oxygen) may be present in minor amounts as an impurity in processes such as those disclosed in application Ser. No. 528,804, either as a feedstock impurity or if elaborate steps are not taken to exclude and fully purge ambient air from the reactor, tubing, etc. Additionally, example 1 of the application used as a feedstock natural gas which contains minor amounts of oxygen as an impurity.
Subsequent to the filing of application Ser. No. 528,804, some researchers, attempting to duplicate the results there disclosed, have experienced difficulty in producing pure diamond films, if they took steps to exclude and purge all air from the reactor system. Other researchers, using similar oxygen-excluding measures, have reported no difficulty in duplicating applicant's results.
As a result of further study and experimentation, the present inventors have determined that the production of substantially pure diamond films, in accordance with the methods of application Ser. No. 528,804, may be accomplished most reliably by the inclusion in the reactant gases of a minor amount of a chalcogen, most preferably oxygen or sulfur.